1. Field of the Invention
This invention relates to a strain detector for detecting strain of a driven shaft such as, for example, a rotary shaft.
2. Description of the Prior Art
An exemplary one of conventional strain detectors is shown in FIG. 7. Referring to FIG. 7, a driven shaft 1 in the form of a rotary shaft which is an object for the detection of strain is supported for rotation around a center axis 2 by means of a pair of bearings 3 and 4. First and second elongated magnetic layers 5 and 6 made of a soft magnetic material having a high permeability and suitable magnetostriction are fixedly mounted on an outer periphery of the driven shaft 1 in a spaced relationship from each other in an axial direction of the driven shaft 1. Each of the magnetic layers 5 and 6 is composed of a plurality of parallel layer stripes, and the stripes of the first magnetic layers 5 extend at an angle of +45 degrees with respect to the center axis 2 while the stripes of the second magnetic layers 6 extend at another angle of -45 degrees with respect to the center axis 2. A cylindrical coil bobbin 7 is disposed around the magnetic layers 5 and 6 in a concentrical relationship to the driven shaft 1. First and second detecting coils 8 and 9 in the form of solenoids are wound on the coil bobbin 7 corresponding to the magnetic layers 5 and 6, respectively. The detecting coils 8 and 9 are connected to a detecting circuit 14. A pair of magnetic converging layers 10 and 11 made of a soft magnetic material having a high permeability and suitable magnetostriction are disposed around the detecting coils 8 and 9, respectively.
With the strain detector of the construction described above, if an external force is applied to the driven shaft 1, then a tensile force is produced on either one of the magnetic layers 5 and 6 while a compression force is produced on the other of the magnetic layers 5 and 6. The magnetostriction of the magnetic layers 5 and 6 allows the orientation of the magnetization within each domain to be altered by the stress associated with applied torque, then the permeability of the magnetic layers 5 and 6 is varied. In this instance, the permeability is varied in the opposite direction whether the strain is caused by a tensile force or a compression force. Each of the detecting coils 8 and 9 detects a variation in permeability as a variation in magnetic impedance, and the detecting circuit 14 detects and amplifies a difference between outputs of the detecting coils 8 and 9 and develops a detection voltage V corresponding to an amount of strain of the driven shaft 1. Each of the magnetic converging layers 10 and 11 converges magnetic fluxes on the outer periphery side of the detecting coil 8 or 9 to reduce the reluctance of the magnetic circuit to improve the sensitivity of the strain detector.
FIGS. 8a and 8b show a magnetic circuit and an electric equivalent circuit of the conventional strain detector described above. Referring to FIGS. 7, 8a and 8b, magnetic fluxes generated upon energization of each of the detecting coils 8 and 9 include magnetic fluxes Fg and F'g which pass through a gap between the detecting coil 8 or 9 and the magnetic layer 5 or 6, magnetic fluxes Feff which pass through the magnetic layer 5 or 6, and magnetic fluxes Fs which pass through the driven shaft 1. Since the magnetic converging layers 10 and 11 have a high permeability, the magnetic fluxes Fg, Feff and Fs all path through the magnetic converging layer 10 or 11 on the outer periphery of the detecting coil 8 or 9. Meanwhile, electric currents Ig, Ieff and Is are obtained by conversion of the magnetic fluxes Fg and F'g, Feff, and Fs, respectively, while a voltage E corresponds to a magnetomotive force of the detecting coil 8 or 9. Further, resistors Rg, Reff and Rs correspond to reluctances of the gap, the magnetic layer 5 or 6, and the driven shaft 1, respectively, while a resistor Rex corresponds to a reluctance on the outer periphery side of the detecting coil 8 or 9. In addition, a reluctance Ry of the magnetic converging layer 10 or 11 is inserted in parallel to the resistor Rex. Thus, since the reluctance Ry has a comparatively low value, the total electric current I is high and the magnetic fluxes Feff are also high, and consequently, the sensitivity is high.
With the conventional strain detector described above, however, the magnetic fluxes Fg, F'g and Fs which do not pass through the magnetic layer 5 or 6 and accordingly do not contribute to detection of strain are involved at a comparatively high ratio. Consequently, the conventional strain detector has a problem that a sufficiently high strain detecting sensitivity cannot be attained.
Besides, with the conventional strain detector described above, while magnetic fluxes generated by each of the detecting coils 8 and 9 pass through the magnetic converging layer 10 or 11 and then through the magnetic layer 5 or 6 to make loops, the distribution of the magnetic fluxes is uneven such that the intensity of the magnetic field at an end portion of the detecting coil 8 or 9 is about one half the intensity of the magnetic field at a central portion of the detecting coil 8 or 9 as seen in FIGS. 9a and 9b, that is, He.apprxeq.(1/2).multidot.Ho, where Ho is an intensity of a magnetic field at a central portion of the length l of the detecting coil 8 or 9, and He is an intensity of a magnetic field at an end portion of the coil 8 or 9. Accordingly, while the magnetic flux density of the magnetic layer 5 or 6 is comparatively high at a central portion of the detecting coil 8 or 9 in the lengthwise direction, part of the magnetic fluxes pass a spacing between the detecting coil 8 or 9 and the magnetic layer 5 or 6 as seen in FIG. 9b. Consequently, the energization efficiency of the detecting coils 8 and 9 are deteriorated and the sensitivity of the strain detector is deteriorated.